Tweak the travis config.

Add Ruby 2.1 as a build target, and replace the deprecated `rbx-19mode`
and `rbx-20mode` targets with `rbx` for Rubinius.
Update the Gemfile for Rubinius.

This adds the `rubysl` gem as a dependency when running on rubinius.
It’s not a dependency of Rugged per se, but adding it to the Gemfile
makes sure our build runs successfully on Travis.

It’s also what other projects, e.g. rails do.
Merge pull request #302 from libgit2/arthur/tweak-travis

Tweak the travis config